A linear motion guide device includes a guide rail extending in a linear shape, and a slider assembled to the guide rail so as to be movable in a longitudinal direction of the guide rail. A raceway groove constituted by a recessed groove extending in the longitudinal direction of the guide rail is formed on a surface of the guide rail, and a raceway groove constituted by a recessed groove opposed to the raceway groove on the guide rail is formed on a surface of the slider. A rolling passage on which balls as rolling elements are to roll is formed between the raceway groove of the guide rail and the raceway groove of the slider and the rolling passage extends in the longitudinal direction of the guide rail. A plurality of balls is rollably disposed in the rolling passage, and the slider is guided by the guide rail so as to be movable through the rolling of the plurality of balls in the rolling passage.
As the raceway groove in such a linear motion guide device, generally, an arcuate groove having a single-arc sectional shape (sectional shape cut on a plane perpendicular to the longitudinal direction of the guide rail), or a gothic-arc groove having a generally V-shaped sectional shape is used. When the gothic-arc groove is employed as the raceway groove, there is such an advantage that accuracy of a dimension, a shape, or the like of the raceway groove is easily secured.
In a linear motion guide device as described in PTL 1, 2, a gothic-arc groove is provided as a raceway groove, and when the linear motion guide device is used, a ball makes contact with only one flank out of two flanks constituting the gothic-arc groove. A sectional shape of either flank is a single-arc shape.
In the meantime, in a linear motion guide device as described in PTL 3, a sectional shape of a flank of a raceway groove is a combined-arc shape in which a plurality of arcs having different curvature radiuses is continuous with each other. With such a configuration, a contact surface pressure between a ball and the raceway groove is reduced, so that a life of the linear motion guide device increases and a coefficient of dynamic friction is reduced.
However, the technique described in PTL 3 is achieved on the premise of a case where a sectional shape of the raceway groove is linearly symmetric across a line of action of a load to be applied to a contact point between the ball and the raceway groove with the line of action of the load being taken as an axis of symmetry. Accordingly, like the linear motion guide device described in PTL 1, 2, in a case where the sectional shape of the raceway groove is not linearly symmetric but asymmetric across a line of action of a load to be applied to a contact point between the ball and the raceway groove with the line of action of the load being taken as an axis of symmetry, even if the technique described in PTL 3 is applied, a contact surface pressure between the ball and the raceway groove might not be reduced sufficiently.
That is, in a linear motion guide device in which a sectional shape of a raceway groove is linearly symmetric across a line of action of a load as an axis of symmetry, in a case where a load (hereinafter referred to as a “pressing load”) directed vertically downward is applied to a slider like a case where an object is put on the slider, in a case where a load (hereinafter referred to as a “tensile load”) directed vertically upward is applied to the slider, or in a case where a load (hereinafter referred to as a “lateral load”) along a guide-rail width direction is applied to the slider, even if the load is applied, a pressure pattern at a contact point between a ball and the raceway groove does not change. However, in a linear motion guide device in which a sectional shape of a raceway groove is asymmetric across a line of action of a load as an axis of symmetry, a pressure pattern changes at the time when the load is applied, and therefore, an effect of reducing the contact surface pressure between the ball and the raceway groove cannot be obtained sufficiently, which might cause an edge load.
More specifically, when a tensile load is applied to the slider, a contact angle increases in comparison with a state where the tensile load has not been applied, so that a range of a contact stress generated in a contacting portion between a flank of the raceway groove and the ball increases, which may cause the range to reach a chamfer provided in a groove shoulder of the raceway groove. When the range of the contact stress reaches the chamfer, an edge load occurs in an edge of the chamfer because the contact stress cannot be received by the chamfer. The edge load is a remarkable pressure peak, and therefore causes plastic deformation of materials constituting the guide rail and the slider. When the raceway groove deforms plastically, smooth circulation of the ball is prevented, and thus, the occurrence of the edge load is unfavorable.
Further, when a lateral load is applied to the slider, the contact angle decreases in comparison with a state where the lateral load has not been applied, so that the range of the contact stress generated in the contacting portion between the flank of the raceway groove and the ball increases, which may cause the range to reach a wire groove (a groove that partially accommodates a cage to prevent interference between the cage and the guide rail) provided on a groove bottom of the raceway groove. Since the wire groove cannot receive a contact stress, an edge load occurs in the edge portion of the wire groove.